Automated testing of biological samples is becoming more prevalant. For example, automated testing apparatus for photometrically determining coagulation time and other blood parameters are disclosed in U.S. Pat. Nos. 3,969,079, 3,607,099, and 3,718,439. In these instruments, a sample is placed into a cuvette, after which the testing instrument automatically dilutes the sample and adds reagents as necessary to perform the test. After the sample has been so treated, the cuvette is moved into position so that an optical beam from a light source passes through the sample and impinges upon a photodetector which provides an output representative of the optical density of the sample. By suitably processing and measuring the photodetector output, various parameters may be determined. The techniques for performing such tests are well known, as exemplified by the above referenced patents.
The design of cuvettes for use in such apparatus is important to achieving optimum performance from the apparatus. Proper design increases the accuracy of the measurement. Operator errors may be minimized by providing the cuvettes with coding to automatically inform the testing instrumentation of the test to be performed.
Many different cuvette designs have been developed over the years. See for example, U.S. Pat. Nos. 3,905,772, 4,119,407, 4,251,159, and 4,371,498. In the last mentioned patent, a cuvette having dual receptacles is described. The cuvette further includes coding to enable a testing instrument to determine the particular test procedure to be performed on the sample in the cuvette. The cuvette is encoded by means of one or more holes placed in the top section of the cuvette. These holes may be sensed by solenoid operated probes in the testing instrument to decode the cuvette. This method, while successful, had drawbacks, including contamination of the probes requiring frequent maintenance, and the necessity of making the cuvette walls thick enough to withstand the force of the mechanical probe.
Due to various considerations, discussed in more detail below, the walls of cuvettes for blood testing must be as thin as practical. Because of this, optically encoding cuvettes has posed problems. Cuvettes are typically made from plastic materials. In order to avoid contaminating samples and increasing costs, it is undesirable to apply extraneous materials, such as labels or reflectors, to the cuvettes. Due to the thinness of the cuvette walls, it is difficult to make sections of a cuvette opaque enough, for example by roughening the surface, to be reliably detected by a photo-optical detector.